Exynos542x: Move exynos5420_get_pll_clk up and rename

Moving exynos5420_get_pll_clk function definition up in the
code to keep it together with rest of SoC_get_pll_clk functions.
This makes code more legible and also removes the need of
declaration when called before the position of definition in
code. Also, renaming exynos5420_get_pll_clk to
exynos542x_get_pll_clk because it is being used for both Exynos
5420 and 5800.

Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>